Material performance testing including improved load detection

ABSTRACT

A material testing apparatus includes an actuator to apply a force to a load head according to electronic control signals. The load head supplies a load to a material specimen in a first dimension. A plurality of load line displacement (LLD) reference points extend radially outward from the load head; and a plurality of LLD measuring devices correspond to the plurality of LLD reference points. Each LLD measuring device is positioned to detect a position of a corresponding LLD reference point along the first dimension and is configured to transmit position signals to a controller programmed to perform a performance test on the material specimen using feedback control based on a combination of the position signals, including an average of the position signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/865,993 filed May 4, 2020, being issued as U.S. Pat. No. 11,119,017on Sep. 14, 2021, which is a continuation of U.S. patent applicationSer. No. 15/864,465 filed Jan. 8, 2018, now U.S. Pat. No. 10,641,690issued May 5, 2020, which claims the benefit of U.S. Provisional Ser.No. 62/443,628 filed Jan. 6, 2017. This disclosure of the priorapplications is considered part of (and is incorporated by reference in)the disclosure of this application.

TECHNICAL FIELD

This specification generally relates to systems, techniques,apparatuses, and devices for improved displacement control of loadingapparatuses used for performance testing of constructions materials,such as asphalt, concrete, masonry, structural steel, soil, and others.

BACKGROUND

Testing protocols and control systems have been developed to test thefracture properties of asphalt. For example, Illinois Test Procedure 405has been developed and published by the Illinois Department ofTransportation for determining the fracture potential of asphaltmixtures using the Illinois Flexibility Index Test (I-FIT). As part ofthis test, an asphalt pavement core or Superpave Gyratory Compactor(SGC) compacted asphalt mixture specimen is trimmed and cut in half tocreate a semicircular shaped test specimen. A notch is sawn in the flatside of the semicircular specimen opposite the curved edge. The specimenis conditioned and maintained through testing at 25° C. (77° F.). Thespecimen is positioned in the fixture with the notched side downcentered on two rollers. A load is applied along the vertical radius ofthe specimen and the loads and Load Line Displacement (LLD) are measuredduring the entire duration of the test. The load is applied such that aconstant LLD rate of 50 mm/min is obtained and maintained for theduration of the test.

In a related standard, the American Association of Street Highway andTransportation Officials (AASHTO) AASHTO Designation TP 124-16 alsodescribes the method of testing for determining the fracture potentialof asphalt mixtures using semicircular bend geometry (SCB) atintermediate temperatures, similar to the I-FIT test.

SUMMARY

This document generally describes systems and control techniques forimproved displacement control of loading apparatuses used forperformance testing of materials, such as for the I-FIT and ASSHTOfracture potential tests. In particular, previous control systems andapparatuses have relied on the use of a singular LLD measurement devicethat is cantilevered off the side of the load head to provide LLDmeasurements, which are then used to control the LLD rate being appliedto the specimen under test and to determine the ultimate flexibilityindex (FI) for the specimen. The FI measurement can indicate how well anasphalt specimen under test can carry a load after it has reached aninitial structural breaking point. To provide accurate, consistent, andreliable FI measurements, a testing apparatus and control system needsto be able to accurately measure displacement of the load head and toaccount for deformation of the head during testing to ensure aconsistent rate of LLD is applied to the specimen. Measurements beingoff by as much as 1/1000″ can cause the FI measurement to be incorrectby significant amounts—potentially providing false positives for failingspecimens.

As described throughout this document, improved displacement control canbe achieved in a variety of ways, such as through the use of multipleLLD measurement devices (e.g., multiple displacement transducers, othersuitable LLD measurement devices) to provide multiple LLD measurementsthat can be combined to calculate a more accurate position and/orposition rate (velocity) for the load head. Control of an actuatordriving the load head can additionally be improved by using the moreaccurate position and/or position rate information, which can providefor more consistent and reliable LLD rate during testing, and ultimatelymore accurate and reliable FI measurements for the sample under test.

Multiple LLD measurements taken at the same time from multipledisplacement transducers can be combined in any of a variety of ways,such as through averaging the values, using weighted averages (e.g.,weighting based on any of a variety of factors), median values, and/orother combinations of values.

Multiple LLD measurement devices, such as multiple displacementtransducers, can be located on opposing or near-opposing sides of aspecimen under test so that bending moments in the load head (and/orother variations in the position of the LLD measurement devices notattributable to the deformation of the specimen under test and/or theload head) can be accounted for and discounted. Such positioningvariations in a LLD measurement device can introduce errors that candramatically alter the LLD rate and the ultimate FI measurement for atest if unaccounted for. For example, with a single cantilevered LLDmeasurement device off of one side of a specimen under test, priorapparatus and control systems (e.g., I-FIT test) may not be able todetect or correct for such positioning variations that are notattributable to the specimen under test or the load head, and mayinstead be interpreted as part of the LLD measurement for the specimenunder test and/or the load head. The disclosed technology improves uponthe accuracy of such control systems and apparatuses by accounting andcorrecting for such variations, which should not be attributed to eitherthe specimen under test or the load head in order to ensure an accurateFI test result. For example, by combining measurements from multiple LLDmeasurement devices (e.g., using the average of multiple displacementtransducers) from particularly selected reference points on a machine,the bias introduced by machine compliance can be canceled, resulting ina more accurate measurement of displacement and loading rate control ofthe specimen under test.

In one implementation, a material testing apparatus includes an actuatorto drive a piston according to electronic control signals; a load headto supply a load to a material specimen in a first dimension, whereinforce is applied to the load head by the piston; a plurality of loadline displacement (LLD) reference points that extend radially outwardfrom the load head; a plurality of LLD measuring devices that correspondto the plurality of LLD reference points, each of the plurality of LLDmeasuring devices (i) being positioned to detect a position of acorresponding LLD reference point along the first dimension and (ii)being configured to transmit position signals to a controller; and aload cell to measure the load supplied to the material specimen by theload head, wherein the load cell is configured to transmit load signalsto the controller.

Such material testing apparatus can optionally include one or more ofthe following features. The plurality of LLD measuring devices caninclude a plurality of transducers. The plurality of LLD referencepoints can include a plurality of magnets. The plurality of transducerscan include a plurality of non-contact magneto restrictive positiontransducers that measure the positions of the plurality of magnets alongthe first dimension. The plurality of magnets can extend radiallyoutward from the load head in a second dimension that is substantiallyperpendicular to the first dimension. The plurality of magnets canextend from opposing sides of the load head. The plurality ofnon-contact magneto restrictive position transducers can be positionedon opposing sides of the material specimen. The material specimen can bean asphalt specimen. The controller can be programmed to perform aflexibility index (FI) test on the asphalt specimen using feedbackcontrol based on (i) a combination of the position signals from theplurality of non-contact magneto restrictive position transducers and(ii) the load signal from the load cell. The controller can provide thecontrol signals to the actuator according to the feedback control sothat a target rate of LLD is achieved during the FI test. The targetrate of LLD can be 50 mm/minute. The combination of the position signalscan include an average of the position signals. The controller can beseparate from the apparatus. The apparatus can further include thecontroller, wherein the controller is programmed to perform aperformance test on the material specimen using feedback control basedon (i) a combination of the position signals from the plurality of LLDmeasuring devices and (ii) the load signal from the load cell, andwherein the controller determines and provides the control signals tothe actuator according to the feedback control so that a target rate ofLLD is achieved during the performance test. The combination of positionsignals can include an average of the position signals.

In another implementation, a material testing system includes anactuator to drive a piston according to electronic control signals; aload head to supply a load to a material specimen in a first dimension,wherein force is applied to the load head by the piston; a plurality ofload line displacement (LLD) reference points that extend radiallyoutward from the load head; a plurality of LLD measuring devices thatcorrespond to the plurality of LLD reference points, each of theplurality of LLD measuring devices (i) being positioned to detect aposition of a corresponding LLD reference point along the firstdimension and (ii) being configured to transmit position signals; a loadcell to measure the load supplied to the material specimen by the loadhead, wherein the load cell is configured to transmit load signals; anda controller configured to (i) perform feedback control of the actuatorbased, at least in part, on the position signals and the load signalsduring a performance test of the material specimen, (ii) record dataduring the performance test, and (iii) determine a result for theperformance test based on the recorded data.

Such a material testing system can optionally include one or more of thefollowing features. The feedback control includes repeatedly performingthe following during the performance test: receive the position signalsfrom the plurality of LLD measuring devices, receive the load signalsfrom the load cell, combine the position signals into a combinedposition, determine an LLD measurement for the material specimen basedon the combined position, compare the LLD measurement with an targetloading rate for the performance test, determine the control signals forthe actuator based on the comparison and the load signals, and providethe control signals to the actuator. Combining the position signals caninclude averaging the position signals and the combined position can bean average position. The performance test can include an FI test. Thematerial specimen can include an asphalt specimen. The data that isrecorded can include the average position and the load signals. Theresult for the FI test can be an FI result value. The target loadingrate can include 50 mm/minute. The plurality of LLD measuring devicescan include a plurality of transducers. The plurality of LLD referencepoints can include a plurality of magnets. The plurality of transducerscan include a plurality of non-contact magneto restrictive positiontransducers that measure the positions of the plurality of magnets alongthe first dimension. The plurality of magnets can extend radiallyoutward from the load head in a second dimension that is substantiallyperpendicular to the first dimension. The plurality of magnets canextend from opposing sides of the load head. The plurality ofnon-contact magneto restrictive position transducers can be positionedon opposing sides of the material specimen. The material specimen caninclude an asphalt specimen. The controller can be programmed to performa flexibility index (FI) test on the asphalt specimen using feedbackcontrol based on (i) a combination of the position signals from theplurality of non-contact magneto restrictive position transducers and(ii) the load signal from the load cell. The controller can provide thecontrol signals to the actuator according to the feedback control sothat a target rate of LLD is achieved during the FI test. The targetrate of LLD can be 50 mm/minute. The combination of the position signalscan include an average of the position signals. The plurality of LLDmeasuring devices can include two LLD measuring devices. The pluralityof LLD measuring devices can include four LLD measuring devices.

In another implementation, a method for performing a performance test ona material specimen includes performing feedback control on a materialtesting apparatus that includes (i) a load head to supply a load to amaterial specimen, (ii) a plurality of load line displacement (LLD)reference points that extend radially outward from the load head, (iii)a plurality of LLD measuring devices to provide position signalsindicating positions of the plurality of LLD reference points, and (iv)a load cell to provide load signals for the load supplied to thematerial specimen, the feedback control including repeatedly performingthe following: receiving the position signals from the plurality of LLDmeasuring devices, receiving the load signals from the load cell,combining the position signals into a combined position, determining anLLD measurement for the material specimen based on the combinedposition, comparing the LLD measurement with an target loading rate forthe performance test, determining the control signals for the actuatorbased on the comparison and the load signals, and providing the controlsignals to the actuator; and determining a result for the materialspecimen under the performance test based on, at least, changes in thecombined position and the load signals during the performance test.

Certain implementations can provide one or more of the followingadvantages. For example, the disclosed technology can improve upon theaccuracy, reliability, and consistency of FI tests that are performed onasphalt samples by reducing measurement error. Additional and/oralternative advantages are also possible, as described below.

BRIEF DESCRIPTION OF THE ATTACHMENTS

FIG. 1 is a conceptual diagram of an example control system forperforming improved displacement control of an example loading apparatusfor performance testing of a material specimen under test.

FIGS. 2A-B are simplified models of the example apparatus experiencing abending moment while the specimen is under test.

FIG. 3 is a block diagram of an example feedback control system for amachine the uses a combination of multiple displacement measuringdevices to control and measure the displacement rate of an actuator thatis used to apply load to a specimen under test.

FIGS. 4A-F are varied views of the example apparatus depicted withmultiple transducers to provide improved material test results.

FIGS. 5A-C present varied views of another apparatus with multipletransducer to provide improved material test results.

FIG. 6 is a graph with example FI test results using the example controlsystems and apparatuses.

FIGS. 7A-C are graphs of example FI test results using the examplecontrol systems and apparatuses.

FIG. 8A depicts an example prior art device that measures displacementusing stroke as the measure of LLD.

FIG. 8B is a graph comparing test results using stroke measurements to acombined displacement value determined from multiple transducers.

FIGS. 9A-E depict an example comparison of average LLD measurementsversus a stroke displacement measurement.

FIG. 10 is a photograph of another material testing apparatus that canuse multiple displacement measuring devices for the load head to controltesting in a feedback loop and to additionally provide improved testingresults.

FIG. 11 is a side view of an example DCT test specimen.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of an example control system 100 forperforming improved displacement control of an example loading apparatus102 for performance testing a material specimen 116 under test. In thedepicted example, the apparatus 102 is performing a test on a SCBspecimen 116, which is a semi-circular shape or half-disk of a material,such as an asphalt mixture. The apparatus 102 is designed to apply aload via a load head 108 to the specimen 116, which can be loaded in athree point bend configuration, for example. The load is applied by anactuator 104 (e.g., hydraulic actuator) which can drive the load head108 along a load column 110 onto the specimen 116. For example, the toploading head 108 can use a pivoting rounded loading strip to contact themiddle of the specimen 116, and force can be applied using a hydraulicactuator (example of actuator 104).

The apparatus 102 includes sensors to provide feedback signals to acontroller 122 that is controlling application of the actuator 104 toprovide, for example, a target LLD rate to the specimen 116 (e.g.,driving the load head 108 at a target LLD rate of 50 mm/min). Thesesensors can include, for example, a load cell 106 to measure the loadthat is being applied to the specimen 116 and multiple transducers 114a-b (example LLD measuring devices) to provide measurements of the LLD.The transducers 114 a-b measure the position of LLD reference pointsthat are defined by reference point devices 112 a-b (e.g., magnets) thatare cantilevered off either side of the load head 108. For example, thetransducers 112 a-b can be non-contact magneto restrictive positionsensors/transducers that measure the position of magnets (examplereference point devices 112 a-b) on the left and right side of theloading fixture 108. In another example, the transducers 112 a-b can becontact-type LVDTs (or other contact-type transducers) that can physicalcontacted by physical objects (example reference point devices 112 a-b)and/or can be placed directly on the specimen 116 to measuredisplacement (instead of or in addition to using reference points 112a-b). In this example, by averaging multiple contact-type sensors on thespecimen 116, a high level of rate controllability and measurement canbe achieved. Other types of position sensors can be used to directlymeasure on the specimen 116, such as extensometers (a type of reusablestrain gauge). In a further example, other types of sensors (e.g.,optical sensors, radiation sensors, vibration sensors, movement sensors,altimeters) can be used as the transducers 112 a-b and correspondingposition indicators (e.g., light sources, radiation sources) can be usedas the reference point devices 112 a-b, where appropriate.

As the load is being applied to the specimen 116, the load cell 106 cantransmit load signals 132 to the controller 122, and the transducers 114a-b can transmit left and right position signals 130 a-b to thecontroller 122. The controller 122, which can be any of a variety ofappropriate computing devices (e.g., embedded controller,application-specific integrated circuit (ASIC), laptop/desktop computer,mobile computing device) that are configured to receive the signals 130a-b and 132, and to control the apparatus 102, can receive the signalsand use them to determine the left and right positions for the head 108,and to combine the determined left and right positions (140). Forexample, the head 108 may have a bending moment 160 that may cause thehead to pivot to the left or to the right. When this happens, each ofthe positions by themselves may inaccurately provide the displacement ofthe loading head 108. However, by combining the positions, theinaccuracies of each position can be canceled out to provide an accuratedisplacement for the head 108. In prior systems that used just a singletransducer, such inaccuracies would not be recognized or eliminated fromthe head 108 displacement determination. The left and right positionscan be combined in any of a variety of ways, such as through averaging,weighted averaging, determining the median value, and/or otherappropriate techniques for statistically combining values.

Using the combined position information, the controller 112 candetermine the displacement (e.g., LLD) for the head 108 and/or thespecimen 116 (142), and can control the loading rate based on thecombined position, the determined displacement, and/or the loadmeasurement from the load cell 106. The loading rate can be determinedso that the LLD rate is consistent and within an acceptable threshold ofa target LLD rate (e.g., 50 mm/min). Control signals 150 can betransmitted to the actuator 104 by the controller 122. The steps 140-142can be continually and repeatedly performed by the controller 122 toconsistently control the loading rate for the sample 116 during thetest.

The controller 122 can additionally record the displacement and loadvalues over time during application of the test in order to calculatethe results for the specimen 116. For example, the controller 122 candetermine the FI for the specimen 116 by analyzing the displacement andload over time.

FIGS. 2A-B are simplified models of the example apparatus 102experiencing a bending moment 160 while the specimen 116 is under test.

Referring to FIG. 2A, in the depicted example model the two loadingcolumns and the single load cell are represented by springs. In somecases, a loading base 120 is supported a support shaft (or load cell),which can also be represented by a spring. The spring stiffness of loadCol1 and Col2 are indicated by the K_(Col1) and K_(Col2), respectively.The load cell and support shaft stiffness is represented by K_(LC) andK_(SS), respectively. As the specimen is loaded, the loading columns,load cell, and support shaft all deflect by a value proportional totheir stiffness. The amount of deflection is represented by the value Xfor each component.

In this example, the deformation of the specimen 160 (as indicated bythe crack 170) is estimated by the relative difference between areference point on the loading head 108 and loading base 120, referredto here as Load Line Displacement (LLD). When a bending moment 160 isinduced on the loading head and loading base, the magnitude of the leftside displacement (LLD_L, 130 a) will not be the same as thedisplacement of the right side displacement (LLD_R, 130 b). Accordingly,if only one of the LLD_L (130 a) or LLD_R (130 b) values were used bythe controller 122, the ultimate FI results determined by the controller122 would be different and inaccurate. By including devices 114 a-b tomeasure both LLD_L and LLD_R, the controller 122 can account for thediffering magnitude on the left and right sides to more accurately andconsistently determine the LLD, and to more accurately determine theultimate FI results for the specimen 116.

Referring now to FIG. 2B, this model shows the exaggerated compliance ofthe two loading columns, single load cell, and support shaft (ascompared with the model in FIG. 2A). The deflection of the loadingcolumns, load cell, and support shaft is not always uniform orpredictable. Since the crack 170 is not uniform, as the specimen 116breaks, a moment 160 (bending force) is induced on the loading framecomponents. This means that Col1 can deflect a different amount thanCol2, and the loading head 108 and/or loading base 120 can compress andpivot. Using any of these elements as reference points in the estimationof the specimen 116 deformation can be introduce measurement bias. Asshown in this example the displacements of LLD_L (130 a) and LLD_R (130b) are not equal. Variation can additionally be introduced throughdifferent loading frames and fixtures that have different stiffness, anddifferent components that deflect differently under load. Therefore, toreduce variability in the test results, the apparatus 102 can beequipped with multiple transducers 114 a-b and the controller 122 can beprogrammed to combine multiple position measurements in order tomitigate the effects of frame and fixture compliance. For example, usingaverage displacement control by the controller 122 can provide acanceling effect of side measurement bias from averaging opposite (inthe case of 2 transducers) or multiple (more than 2 transducers) sidesof the loading fixture.

FIG. 3 is a block diagram of an example feedback control system 300 fora machine (e.g., apparatus 102) the uses a combination of multipledisplacement measuring devices (e.g., transducers) to control andmeasure the displacement rate of an actuator that is used to apply loadto a specimen under test. The system 300 applies to the control ofposition and/or position rate (using differentiated position).

The system 300 includes an example control element 302, such as anembedded digital controller, that runs a real-time operating system. Thecontrol element 302 can include, for example, PC hardware (e.g.,processor, microprocessor, memory) that is part of the embeddedcontroller and that is responsible for running the control algorithms,data acquisition, and signal outputs in both digital and analog form.For example, the controller 122 is one example of the control element302.

The system 300 also includes an actuation element 304 that can createlinear motion for a loading head. The actuation element 304 can be, forexample, a hydraulic actuator, such as the actuator 104. Movement of ahydraulic actuator can be fed, for example, by a hydraulic pump througha hydraulic servo-valve (a flow control device). Other implementationsof the actuation element 304 are also possible, such as mechanical screwdrives and/or other load driving devices.

The actuation element 304 drives a loading element 306, which caninclude a loading frame and specialized fixtures for each specific test.For example, the loading element 306 can include a loading head (e.g.,loading head 108) attached to the end of the piston rod (through a loadcell). The hydraulic cylinder can be held in place through load columns(e.g., 2 columns, 4 columns) attached to a top cross-member and bottombase plate. The base of the loading fixture can include additionalpoints of contact with the specimen under test (e.g., specimen 116).Other configurations of the loading element 306 are also possible.

The loading element 306, however, can be susceptible to disturbances,which are errors resulting from machine compliance that includes, butnot limited to, stretching of the loading columns, compression of theload cell, compression of the hydraulic piston seals, bending of thefixture caused by non-uniform breaking of the specimen under test,and/or combinations thereof. Each element of the loading fixture can bethought of as a spring with different stiffness. In fact, a load cellcan be, by definition, a spring device. Since each machine can have adifferent stiffness, each machine can have a different magnitude ofmeasurement and control error. The systems 100 and 300 can eliminate theuncertainty caused by such disturbances through control and measurementtechniques that leverage multiple position measurements to cancel theeffects of errors by combining (e.g., averaging) the multiplemeasurements to mitigate the effect of such disturbances.

The loading element 306 can apply the load to a specimen under test 308,which can be an asphalt sample, which is a viscoelastic material.Viscoelastic materials have a unique property in which the stiffness ofthe material is a function of the displacement rate.

Position measurements can be determined for the system 300 by multipletransducers 310 a-n, which can include any number of transducersmeasuring position at independent locations relative the specimen undertest 308 (e.g., measuring on opposing sides of the specimen 308). Thetransducers 310 a-n can measure the position of elements (e.g.,reference point magnets) that extend from the loading element 306 (e.g.,extend perpendicular to the direction of linear displacement). Theposition elements can be configured so that they provide readilycomparable position measurement values, such as through extending thesame or similar distance from a common point (e.g., center point) on theloading element 306. The transducers 310 a-n can be, for example,non-contact type, magneto restrictive position sensor, such as BALLUFF,model BTL001W. Other possible transducers 310 a-n can include any formof linear variable differential transformer (LVDT). Types of LVDTs caninclude contact type and/or non-contact type transducers. In anotherexample, a rotary variable differential transformer (RVDT)/motor encodertypically used on a mechanical screw type actuator could additionallyand/or alternatively be used.

The position measurements from the transducers 310 a-n can be providedto a combining element 312, which can be, for example, a routine on acontroller (e.g., software on embedded controller) that combines thesignals (e.g., averages the signals) and maps the combined position toits own unique feedback channel that is used by the control element 302.The feedback control loop including the control element 302, theactuation element 304, the loading element 306, the transducers 310 a-n,and the combining element 312 can be used to continually control theloading rate applied to the specimen 308 so that a target rate isachieved, and to record displacement and loading measurements todetermine an ultimate test result for the specimen 308 (e.g., FI testresult).

FIGS. 4A-F are varied views of the example apparatus 102 depicted inFIG. 1 with multiple transducers 114 a-b to provide improved materialtest results. FIG. 1 presents a front view of the apparatus 102. FIGS.4A-B present angled views of the apparatus 102. FIGS. 4C-D presentvaried perspective views of the apparatus 102. FIG. 4E presents a topview of the apparatus 102. FIG. 4F presents a perspective view of theapparatus 102 without the specimen 116.

FIGS. 5A-C present varied views of another apparatus 500 with multipletransducer 502 a-b to provide improved material test results. In thedepicted examples, the apparatus 500 is configured to perform tests oncircular material samples, such as asphalt. The apparatus 500 can beused in combination with a controller to perform improved materialtesting, as described above with regard to FIGS. 1-4 . FIG. 5A presentsa perspective view of the apparatus 500. FIG. 5B presents a front viewof the apparatus 500. FIG. 5C presents another perspective view of theapparatus 500. As described below with regard to FIG. 10 , the apparatus500 can be used to perform indirect tension testing (IDT).

FIG. 6 is a graph with example FI test results using the example controlsystems and apparatuses described above with regard to FIGS. 1-5 . Thisexample shows variability between transducer 1 and transducer 2. In thiscase, transducer 1 was mounted on the “left” side of the machine andtransducer 2 was mounted on the “right” side of the machine. The averagedisplacement (example combined displacement) is a better representationof the actual displacement of the specimen through the specimencenterline.

By using the average displacement to control the test, more accurate andbetter control of the specimen displacement rate can be achieved. Forexample, the desired target displacement rate can be 50 mm/min. In thisexample, the displacement rate for transducer 1 was 49.54 mm/min and thedisplacement rate for transducer 2 was 59.54 mm/min. However, using theaverage displacement to control actuation of the load, a better and moreaccurate rate of 50.04 mm/min was achieved—much closer to the targetrate of 50 mm/min. Since the example FI test depicted uses displacementrate as a means to simulate temperature (using the concept oftime-temperature superposition for viscoelastic materials), accurate andconsistent rate control is as equally important as temperature controlto providing reliable and consistent test results. Variability in rate(as well as temperature) can lead to variability in test results.

FIGS. 7A-C are graphs of example FI test results using the examplecontrol systems and apparatuses described above with regard to FIGS. 1-5. FIG. 7A depicts the results if only the left transducer (e.g.,transducer 114 a) were to be used to perform the test. FIG. 7B depictsthe results if only the right transducer (e.g., transducer 114 b) wereto be used to perform the test. FIG. 7C depicts the results using acombination of both the left and right transducers (e.g., transducers114 a-b) to perform the test. Since shape of the plot is used incalculating test results, variably between left and right measurementsleads to variable in calculated results. Using the previous example, aflexibility index (FI) is calculated for the left, right, and averagevalues. Here each side (FIGS. 7A-B) has an 18% difference in flexibilityindex from the average (FIG. 7C).

FIG. 8A depicts an example prior art device that measures displacementusing stroke as the measure of LLD. One might ask, if measuring off acantilevered point induces error, then why don't we measure off of thecenterline of the actuator? The position of the actuator piston is oftenreferred to as the “Stroke” measurement. Generally, the reference pointof the actuator is a large distance away from the specimen. Since thedeflection of any spring is a function of its length, the longer thedistance from the specimen, the more deflection that is unaccounted forin the estimate of specimen deflection. Additionally, measuring theposition the actuator does not take into account the deflection of theloading columns, load cell and support shaft. From example devicedepicted in FIG. 8A, as the piston compresses the specimen, the loadcolumns will stretch, and the load cell and support column willcompress. This leads to error in the specimen deformation measurement,as well as error in the control of the loading velocity.

Cost can be another issue with centerline actuator displacementmeasurements. Internal LVDTs, such as the one represented in FIG. 8A,inside a hydraulic actuator can be very expensive—much more expensivethan the transducers and other position measuring devices describedabove with regard to FIGS. 1-7 .

FIG. 8B is a graph comparing test results using stroke measurements, asdescribed in FIG. 8A, to a combined displacement value determined frommultiple transducers, as described in FIGS. 1-7 above. This exampleshows that, in some cases, there is very little variability betweentransducer 1 (LLD_L) and transducer 2 (LLD_R). In this case, transducer1 was mounted on the “left” side of the machine and transducer 2 wasmounted on the “right” side of the machine. This test also shows howthere is typically more deflection measured at the actuator piston;measurement of the actuator piston is referred to as its “stroke.” Thisis why the Load Line Displacement (LLD) measurement is more accurate forspecimen testing than the Stroke measurement, even though stroke isalong the centerline of the fixture.

FIGS. 9A-E depict an example comparison of average LLD measurementsversus a stroke displacement measurement. FIGS. 9A-C depict FImeasurements for a left transducer (FIG. 9A), a right transducer (FIG.9B), and an average of the two (FIG. 9C).

Using the previous example, in some cases there may not a significantdifference between results calculated from a Left and Right transducer.In this case, the left and right flexibility index (FI) was within 1% ofthe average. Since the amount of bias is not always predictable, it isnot reliable to assume that test results are unbiased; the only certainway to determine the bias is to measure it. The way to remove the biasis to cancel it out by combining the values, such as through averagingthem.

FIGS. 9D-E provide a comparison of the average LLD determination (FIG.9D) and measurements from an actuator stroke (FIG. 9E). The measurementof actuator stroke is not as reliable as Load Line Displacement (LLD).In the previous example, despite the fact that the results from the leftand right LLD measurements were consistent with the average, themeasurement of stroke was not consistent. The additional deflectionpicked up in the stroke measurement resulted in a 14% reduction inflexibility index (FI). Since a higher flexibility index value isdesirable, measuring stroke falsely degrades the calculated results.

FIG. 10 is a photograph of another material testing apparatus that canuse multiple displacement measuring devices for the load head to controltesting in a feedback loop and to additionally provide improved testingresults. In the depicted example, an indirect tension (IDT) testingmachine is depicted, which is somewhat different from the SCB machinesdescribed above. In particular, with the IDT machine the specimen is afull disk instead of a half-disk like on the SCB machine. The frameconsists of four posts for increased stiffness due to higher loadcapacity. The load cell is mounted to the base plate instead of theactuator rod. A single top and bottom loading strip of equal design areused to contact the specimen. The displacement transducers are mountedto the load cell, and the reference magnets are mounted to the sides ofthe loading head. Examples of an IDT machine are depicted above withregard to FIGS. 5A-C.

The features described above can be applied to other testing machinesand control systems. For example, in addition to averaging values on twoopposing sides of the loading fixture, a testing apparatus could includethree, four, five, six, and/or other numbers of transducers providingvalues that are combined (e.g., averaged). If the bias in a loadingframe and/or specimen is complex enough, additional transducers andreference locations may provide improved controllability and estimationof the deformation of the specimen under test. With three or moretransducers, the bending movement can be resolved along more than oneaxis relative to the specimen. The techniques, systems, apparatuses, anddevices described in this document can additionally be applied toperformance tests that load the specimen in tension, and/or measure thedeformation of the specimen through the use of displacement transducersand/or reference points mounted directly on the specimen. For example,the Disk-shaped Compact Tension (DCT) test (e.g., described by the ASTMD7313 standard) applies a tensile load to the specimen and uses a singleclip-on displacement (COD) gauge mounted on the specimen to control andmeasure the rate of displacement at the specimen's crack mouth. Thefeatures described above can be applied to a DCT test in a variety ofways, such as through the inclusion and use of multiple displacementmeasuring devices (e.g., multiple displacement gauges). For example, aDCT test can be modified to include two displacement gauges that aremounted closer to the crack tip on both (left and right) sides of thespecimen (see FIG. 11 for example side view of a DCT test specimen). Anynon-uniform loading resulting from the interaction between the specimenand the tensile load fixtures, can lead to variability between themeasurement on either side of the specimen. For improved controllabilityand measurement, the techniques described above can be applied tocombine the measurements from these multiple displacement gauges togenerate a more accurate combined measurement. Other variations andapplications of the disclosed techniques, systems, apparatuses, anddevices can be applied to DCT tests, other tensile load tests, and/orother compression tests. While this specification contains many specificimplementation details, these should not be construed as limitations onthe scope of any inventions or of what may be claimed, but rather asdescriptions of features specific to particular implementations ofparticular inventions. Certain features that are described in thisspecification in the context of separate implementations can also beimplemented in combination in a single implementation. Conversely,various features that are described in the context of a singleimplementation can also be implemented in multiple implementationsseparately or in any suitable sub-combination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a sub-combination or variation ofa sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims. In some cases, the actions recited in the claims can beperformed in a different order and still achieve desirable results. Inaddition, the processes depicted in the accompanying figures do notnecessarily require the particular order shown, or sequential order, toachieve desirable results. In certain implementations, multitasking andparallel processing may be advantageous.

What is claimed is:
 1. A material testing apparatus, the apparatuscomprising: an actuator to apply a force; a load head to supply a loadto a material specimen, wherein a flat side of the material specimen ispositioned on two rollers before supplying the load to the materialspecimen; one or more load line displacement (LLD) reference points thatextend outward from the load head; and one or more measuring devicesthat correspond to the plurality of LLD reference points, eachpositioned to detect a position and to transmit position signals to acontroller; and wherein the controller is programmed to perform aperformance test on the material specimen.
 2. The material testingapparatus of claim 1, wherein the measuring devices comprise a pluralityof transducers.
 3. The material testing apparatus of claim 2, furthercomprising a plurality of magnets along a first dimension.
 4. Thematerial testing apparatus of claim 3, wherein the plurality of magnetsextend at least partially radially outward from the load head.
 5. Thematerial testing apparatus of claim 3, wherein magneto restrictiveposition transducers are positioned on opposing sides of the materialspecimen.
 6. The material testing apparatus of claim 3, wherein thematerial specimen comprises a solid specimen.
 7. The material testingapparatus of claim 1, wherein the controller is programmed to perform aflexibility test on the asphalt specimen using feedback control.
 8. Thematerial testing apparatus of claim 7, wherein the target rate of LLD ismeasured in mm/minute.
 9. The material testing apparatus of claim 7,wherein the position signals comprise an average.
 10. The materialtesting apparatus of claim 1, wherein the controller is separate fromthe apparatus.
 11. The material testing apparatus of claim 1, whereinthe controller performs a flexibility index test.
 12. The materialtesting apparatus of claim 1, wherein the controller provides thecontrol signals so that a target rate of LLD is achieved.
 13. A materialtesting system, the system comprising: an actuator to apply a force; aload head to supply a load to a material specimen, wherein a flat sideof the material specimen is positioned on two rollers before supplyingthe load to the material specimen; a plurality of load line displacement(LLD) reference points that extend outward from the load head; a loadcell to measure the loads; and a controller configured to receiveposition signals and load signals from the load cell, combine theposition signals, determine a measurement for the material specimen, andcompare the measurement with a target for a performance test.
 14. Thematerial testing system of claim 13, wherein the material specimencomprises a solid.
 15. The material testing system of claim 14, whereinthe target loading rate is measured in mm/minute.
 16. A method ofperforming a performance test on a material specimen, the methodcomprising: supplying a load to a material specimen; measuring the loadsupplied to the material specimen; generating a load signal based on themeasured load; detecting a displacement, caused by the supplied load, ofat least a portion of the material specimen; generating a positionsignal based on the detected displacement; determining a characteristicof the material specimen using a combination of the position signal andthe load signal, positioning a flat side of the material specimen on tworollers before supplying a load to the material specimen.
 17. The methodof claim 16, wherein detecting a displacement comprises using adisplacement transducer positioned along a side of the materialspecimen.
 18. The method of claim 16, wherein applying the load to amaterial specimen comprises applying the load to a curved edge of thematerial specimen opposite the flat side.
 19. The method of claim 16,wherein positioning the flat side of the material specimen on two therollers comprises centering a notch in the flat side relative to the tworollers.